This invention relates to a vibration damper for vehicle brakes which can prevent squeals of a vehicle wheel brake produced during braking.
A wheel brake tends to squeal during braking. Various means have been proposed to prevent such brake squeals. As conventional such squeal-preventive means, brake vibration dampers disclosed in unexamined Japanese patent publications 4-54324 and 4-54325 are known.
The former publication discloses a vibration damper having a piezoelectric vibrator for applying inaudible vibration to friction members (pads), and another vibration damper comprising a piezoelectric detector for detecting vibration of the friction members, and a piezoelectric vibrator for applying inaudible vibration to the pads only while the detector is detecting vibration of the pads.
The latter publication discloses a vibration damper including a piezoelectric vibration detector for detecting vibration of the friction members, and a vibrator means for vibrating the friction members to dampen their vibration according to the detected vibration. In one embodiment, the vibrator applies vibration 180 .degree. out of phase from the vibration detected. In another embodiment, both the above two piezoelectric elements are used.
The vibration damper systems disclosed in the above two publications apply inaudible vibrations through piezoelectric elements, or vibrate friction members through piezoelectric elements according to the vibration detected to dampen vibration. These systems have various problems.
For example, in the case of a disk brake, in which braking force is applied to the disk by pressing pads as friction members against the disk, if braking force is applied suddenly, continuously or repeatedly for a long time, the pads are heated to high temperatures due to friction heat. Thus, the piezoelectric element, too, provided in the caliper near the pads are heated.
Piezoelectric elements are polarized to produce electric characteristics when pressure is applied to a crystal of e.g. piezoelectric element ZnO.sub.2, and produce piezoelectricity when the above process is reversed. It is preferable to use piezoelectric vibration detector and vibrator having as large a piezoelectric effect (piezoelectric constant) as possible.
But the greater the piezoelectric constant of a piezoelectric element, the lower its Curie temperature and the more it is likely to deteriorate at high temperature. Curie temperature is a critical temperature at which a piezoelectric element loses its spontaneous polarization properties and thus its piezoelectricity. Once a piezoelectric element loses its piezoelectricity, it cannot restore its original properties simply by cooling down to normal temperatures.
The temperature around piezoelectric elements mounted in a brake caliper can rise to 200.degree. C. or higher. If the ambient temperature approaches or exceeds the Curie temperature of the piezoelectric elements, their performance deteriorates quickly. But it is known that piezoelectric elements that have deteriorated in performance and thus have collapsed orientation of polarization can restore polarization orientation by applying a DC high field.
But in spite of this fact, none of the above two publications and any other publication discloses or suggests any solution to the problem of deterioration of piezoelectric elements in brake vibration damper systems.
An object of this invention is to provide a vibration damper system which can restore the performance of piezoelectric elements that have deteriorated in performance due to temperature buildup resulting from repeated braking actions.